The present invention relates to a mesoporous gel comprising an alumina matrix in which one or more oxides selected from silica, boron oxide, phosphorus oxide, oxides of metals from groups VIII and VIB, may be uniformly dispersed. The present invention also relates to a process for the preparation of this gel and its uses as carrier, as catalyst for acid-catalyzed reactions and as hydrotreating catalyst.
The possibility of synthesizing an amorphous alumino-silicate with a high silica content (molar ratio SiO2/Al2O3=70-100) and with a high surface area, characterized by a limited distribution of mesoporosity (with pore dimensions within the range of 37 xc3x85 and 150 xc3x85), is known in literature (M. R. Manton and J. C. Davidtz, Journal of Catalysis (1979), Vol. 60, pages 156-166). These materials have not however found practical applications. More recently, European patent 463.673 and U.S. Pat. Nos. 4.992.608 and 5.049.536 describe the possibility of obtaining amorphous alumino-silicates with a high silica content (SiO2/Al2O3=30-500) with a rather limited distribution of the pore dimensions (average diameter about 10 xc3x85 or less, basic absence of pores with a diameter of more than 30 xc3x85) and excellent catalytic properties in acid-catalyzed reactions.
Even more recently, Italian patent application MI 93 A 002696 describes amorphous alumino-silicates with a high silica content characterized by a dimensionally very limited distribution of the micro-mesoporosity, obtained by a process in which the gelification is carried out at the boiling point (or a little higher) of the alcohols produced by the hydrolysis of the reagents, without eliminating the alcohols themselves from the reagent mixture.
Patent application WO 91/11390 discloses the possibility of obtaining a new group of mesoporous alumino-silicates with a high silica content, called M41S, having an average pore diameter of between 20 xc3x85 and 100 xc3x85 and regularly organized in the material according to a hexagonal (MCM-41) or cubic (MCM-48) structure.
The product obtained is characterized by an X-ray diffraction spectrum from powders (XRD) leading to a hexagonal structure with a bidimensional order or to a structure with cubic symmetry. Analyses carried out via high resolution transmission electron microscopy (HREM) show, in the case of mesoporous silico-aluminates with a hexagonal symmetry (called MCM-41), the presence of monodimensional mesoporosity regularly organized according to a honeycomb hexagonal structure.
In Italian patent application Mi 94 A 01399 it has been found that it is possible to produce micro-mesoporous metal-silicates, with a ratio SiO2/Al2O3 of more than 20, characterized by a narrow distribution of the pore dimensions and with a partial order of these. In fact, whereas the alumino-silicates obtained with the process described in the above Italian patent application Mi 93A002696 are completely amorphous, these new materials called ERS-8, are characterized by an X-ray diffraction spectrum (XRD) from powders which has a diffuse reflection at low angles, indicating a short-range order of the mesoporous structure. The existence of a short-range structural order allows this group of materials to be identified as intermediates among analogous orderly materials (M41S) and completely disorderly materials (amorphous alumino-silicates).
The necessity of having materials with a high surface area, a high pore volume with controlled dimensions is not however only limited to silicas.
At present for example there is a particular interest in hydrotreating catalysts which mostly consist of metals belonging to groups VIB and VIII, supported on alumina (A. B. Stiles, xe2x80x9cCatalysts manufacture, laboratory and commercial preparationxe2x80x9d, Dekker, N.Y., 1983; B. Delmon, xe2x80x9cStudies in surface science and catalysisxe2x80x9d, vol. 53, 1989, page 1-40). For these hydrotreating catalysts of heavy charges it now seems evident, on the basis of information obtained in this field, that the following characteristics are desirable:
a limited microporous fraction (d less than 20 xc3x85) to minimize shape-selectivity phenomena;
a pore distribution not centered in the macroporous region (d less than 500 xc3x85), as the consequent low surface area would be incompatible with a good dispersion of the active phase.
At present these catalysts are prepared for impregnating the carrier with the above metals, in excess or deficiency of the solvent. Gamma-alumina, characterized by a surface area of 200-250 m2/g and an enlarged distribution of the pore diameter centered on 100 xc3x85, is the most widely-used carrier at the moment. Its surface area is further reduced by occlusion during the impregnation processes.
The Applicant has now surprisingly found a new group of materials which have improved characteristics compared to the catalysts of the prior art. These materials are mesoporous gels which comprise an alumina matrix in which one or more oxides selected from silica, boron oxide, phosphorus oxide, oxides of metals of groups VIII and VIB, may be uniformly dispersed, characterized by a controlled distribution of the porosity, a high surface area and a high dispersion of the above oxides, when present.
These materials, depending on their chemical composition, can be well used in various applications. For example, they can be used as mesoporous carriers with a controlled porosity for catalysts, as catalysts for acid-catalyzed reactions and as hydrotreating catalysts. In particular for example gels consisting of alumina alone or gels comprising alumina and one or more oxides selected from silica, boron oxide and phosphorus oxide, preferably silica, can be very suitably used as a carrier for catalysts. In the latter case the presence of these oxides makes the materials with this composition also appropriate as catalysts for acid-catalyzed reactions.
The gels of the present invention which comprise alumina, metal oxides of group VIB and/or VIII, and possibly one or more oxides selected from silica, boron oxide and phosphorus oxide, preferably silica, have a high stability as hydrotreating catalyst.
A first object of the present invention therefore relates to a mesoporous gel comprising an alumina matrix in which one or more oxides selected from silica, boron oxide, phosphorus oxide, the oxide of a metal of group VIII and/or VIB having general formula MOx, may be uniformly dispersed, with the following molar ratios between said oxides and the alumina:
SiO2/Al2O3=0-3.0
B2O3/Al2O3=0-4.0
P2O5/Al2O3=0-0.2
MOx/Al2O3=0-0.2
with a surface area of between 260 and 700 m2/g, with a pore volume of between 0.2 and 1.0 cm3/g and with an average pore diameter of between 20 and 70 xc3x85.
The surface area is preferably between 400 and 700 m2/g and the pore diameter between 20 and 60 xc3x85. According to a preferred aspect of the present invention, the gel consists of alumina alone or comprises alumina and one or more oxides selected from silica, boron oxide and/or phosphorus oxide, preferably silica. According to another preferred aspect of the present invention, the gel comprises alumina, oxides of metals of group VIB and/or VIII, and possibly one or more oxides selected from silica, boron oxide and/or phosphorus oxide, preferably silica. The metal of group VIB is preferably Mo, that of group VIII is preferably Ni.
The X-ray diffraction spectrum from powders (XRD) of these mesoporous gels (called TEG), recorded by means of a Philips vertical diffractometer, equipped with a proportional pulse counter, divergent and receiving sleds of 1/6xc2x0 and with CuKxcex1 radiation (xcex=1.54178 xc3x85), may have an enlarged diffraction line, or however a diffuse scattering, at angular values of not more than 2xcex8=5xc2x0, which can be interpreted with the presence of a short-range order of the mesoporous structure, with a limited structural correlation basically only at the first neighbours, whereas weak and enlarged reflections can always be observed in the high angular region, indicating the incipient formation of a crystalline phase of the y-alumina type.
This mesoporous gel is characterized by a surface area of between 260 m2/g and 700 m2/g, determined with the B.E.T. method by adsorption-desorption cycles of N2 at the temperature of liquid nitrogen (77 K) using a Carlo Erba Sorptomatic 1900 instrument, and by a pore volume of between 0.2 cm 3/g and 1.0 cm3/g. The pore diameter is between 70 and 20 xc3x85, and using the terminology suggested by IUPAC xe2x80x9cManual of Symbols and Terminologyxe2x80x9d (1972), Appendix 2, Part I Coll.Surface Chem. Pure Appl. Chem., Vol. 31, page 578, in which micropores are defined as pores with a diameter of less than 20 xc3x85 and mesopores as those with a diameter of between 20 xc3x85 and 500 xc3x85, this gel has been classified as a mesoporous solid.
The gels of the present invention can be obtained by a process which comprises:
(a) preparing a solution, in an alcohol having the formula Rxe2x80x9cOH, wherein Rxe2x80x9d is a linear or branched C1-C5 group of an aluminium alkoxide having the formula Al (OR)3, wherein R is a linear or branched C1-C5 alkyl group, and possibly of a source of one or more elements selected from silicon, boron, phosphorus, and/or a compound soluble in an alcohol environment of a metal of group VIII;
(b) subjecting the alcohol solution prepared in step (a) to hydrolysis and gelification, at a temperature of between 20 and 80xc2x0 C., with a water solution containing a hydroxide of tetralkylammonium having the formula Rxe2x80x24Nxe2x80x94OH, wherein Rxe2x80x2 is a linear or branched C2-C7 alkyl group, and possibly a compound of a metal of group VIB soluble in a basic environment, the quantity of the constituents of the mixture thus obtained being such as to respect the following molar ratios:
(c) subjecting the gel obtained in step (b) to drying and calcination.
The silica source can be selected from tetra-alkylorthosilicates having the formula Si(ORxe2x80x2xe2x80x3)4, wherein Rxe2x80x2xe2x80x3 is a C1-C3 alkyl group, the boron source can be selected from boric acid and trialkylborates having the formula B(ORiv)3 wherein Riv can be for example ethyl or propyl, the phosphorus source can be selected from phosphoric acid and soluble phosphorus salts.
The compounds soluble in an alcohol environment of metals of group VIII can be for example acetylacetonates.
The compounds soluble in a basic environment of metals of group VIB can be selected from all those which are soluble in a water environment, for example (NH4)6Mo7O24.4H2O.
In the embodiment of step (b) of the above process the order of addition of the two solutions is practically indifferent. After this addition, there is an increase in the viscosity of the reagent mixture, at a rate depending directly on the temperature and composition of the mixture itself, until the formation of a gel. The gelification process is completed in a time of between 1 minute and 3 hours. The gel can be subsequently subjected to aging for a time of between 1 and 72 hours, at a temperature of between 20xc2x0 C. and 80xc2x0 C.
In step (c) of the above process, the gel obtained in step (b) is dried at a temperature of between 50 and 60xc2x0 C. under vacuum and finally calcined in air at a temperature of between 450xc2x0 C. and 550xc2x0 C. for 6-8 hours.
As already mentioned, the gels of the present invention can be suitably used, depending on their chemical composition, as carriers for catalysts, as acid catalysts, when they contain one or more oxides selected from silica, boron oxide and phosphorus oxide, as hydrotreating catalysts, when they contain metals of groups VIB and VIII. In the latter case, they can also, for example, be subjected to impregnation with water solutions of compounds of metals of groups VIII and VIB.